/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.spark.util.collection.unsafe.sort;

import com.google.common.annotations.VisibleForTesting;

import org.apache.spark.memory.MemoryConsumer;
import org.apache.spark.memory.TaskMemoryManager;
import org.apache.spark.unsafe.Platform;
import org.apache.spark.unsafe.array.LongArray;
import org.apache.spark.unsafe.memory.MemoryBlock;
import org.apache.spark.util.Utils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.io.IOException;
import java.util.LinkedList;
import java.util.Queue;

import javax.annotation.Nullable;

import io.indexr.query.TaskContext;

/**
 * External sorter based on {@link UnsafeInMemorySorter}.
 */
public final class UnsafeExternalSorter extends MemoryConsumer {

    private final Logger logger = LoggerFactory.getLogger(UnsafeExternalSorter.class);

    private final PrefixComparator prefixComparator;
    private final RecordComparator recordComparator;
    private final TaskMemoryManager taskMemoryManager;

    /**
     * Memory pages that hold the records being sorted. The pages in this list are freed when
     * spilling, although in principle we could recycle these pages across spills (on the other hand,
     * this might not be necessary if we maintained a pool of re-usable pages in the TaskMemoryManager
     * itself).
     */
    private final LinkedList<MemoryBlock> allocatedPages = new LinkedList<>();

    private final LinkedList<UnsafeSorterSpillWriter> spillWriters = new LinkedList<>();

    // These variables are reset after spilling:
    private volatile UnsafeInMemorySorter inMemSorter;

    private MemoryBlock currentPage = null;
    private long pageCursor = -1;
    private long peakMemoryUsedBytes = 0;
    private volatile SpillableIterator readingIterator = null;

    public static UnsafeExternalSorter createWithExistingInMemorySorter(
            TaskMemoryManager taskMemoryManager,
            TaskContext taskContext,
            RecordComparator recordComparator,
            PrefixComparator prefixComparator,
            int initialSize,
            long pageSizeBytes,
            UnsafeInMemorySorter inMemorySorter) throws IOException {
        UnsafeExternalSorter sorter = new UnsafeExternalSorter(
                taskMemoryManager,
                taskContext,
                recordComparator,
                prefixComparator,
                initialSize,
                pageSizeBytes,
                inMemorySorter);
        sorter.spill(Long.MAX_VALUE, sorter);
        // The external sorter will be used to insert records, in-memory sorter is not needed.
        sorter.inMemSorter = null;
        return sorter;
    }

    public static UnsafeExternalSorter create(
            TaskMemoryManager taskMemoryManager,
            TaskContext taskContext,
            RecordComparator recordComparator,
            PrefixComparator prefixComparator,
            int initialSize,
            long pageSizeBytes) {
        return new UnsafeExternalSorter(
                taskMemoryManager,
                taskContext,
                recordComparator,
                prefixComparator,
                initialSize,
                pageSizeBytes,
                null);
    }

    private UnsafeExternalSorter(
            TaskMemoryManager taskMemoryManager,
            TaskContext taskContext,
            RecordComparator recordComparator,
            PrefixComparator prefixComparator,
            int initialSize,
            long pageSizeBytes,
            @Nullable UnsafeInMemorySorter existingInMemorySorter) {
        super(taskMemoryManager, pageSizeBytes);
        this.taskMemoryManager = taskMemoryManager;
        this.recordComparator = recordComparator;
        this.prefixComparator = prefixComparator;

        if (existingInMemorySorter == null) {
            this.inMemSorter = new UnsafeInMemorySorter(
                    this, taskMemoryManager, recordComparator, prefixComparator, initialSize);
        } else {
            this.inMemSorter = existingInMemorySorter;
        }
        this.peakMemoryUsedBytes = getMemoryUsage();

        // Register a cleanup task with TaskContext to ensure that memory is guaranteed to be freed at
        // the end of the task. This is necessary to avoid memory leaks in when the downstream operator
        // does not fully consume the sorter's output (e.g. sort followed by limit).
        taskContext.addTaskCompletionListener(t -> this.cleanupResources());
    }

    /**
     * Marks the current page as no-more-space-available, and as a result, either allocate a
     * new page or spill when we see the next record.
     */
    @VisibleForTesting
    public void closeCurrentPage() {
        if (currentPage != null) {
            pageCursor = currentPage.getBaseOffset() + currentPage.size();
        }
    }

    /**
     * Sort and spill the current records in response to memory pressure.
     */
    @Override
    public long spill(long size, MemoryConsumer trigger) throws IOException {
        if (trigger != this) {
            if (readingIterator != null) {
                return readingIterator.spill();
            }
            return 0L; // this should throw exception
        }

        if (inMemSorter == null || inMemSorter.numRecords() <= 0) {
            return 0L;
        }

        logger.info("Thread {} spilling sort data of {} to disk ({} {} so far)",
                Thread.currentThread().getId(),
                Utils.bytesToString(getMemoryUsage()),
                spillWriters.size(),
                spillWriters.size() > 1 ? " times" : " time");

        // We only write out contents of the inMemSorter if it is not empty.
        if (inMemSorter.numRecords() > 0) {
            final UnsafeSorterSpillWriter spillWriter =
                    new UnsafeSorterSpillWriter(inMemSorter.numRecords());
            spillWriters.add(spillWriter);
            final UnsafeSorterIterator sortedRecords = inMemSorter.getSortedIterator();
            while (sortedRecords.hasNext()) {
                sortedRecords.loadNext();
                final Object baseObject = sortedRecords.getBaseObject();
                final long baseOffset = sortedRecords.getBaseOffset();
                final int recordLength = sortedRecords.getRecordLength();
                spillWriter.write(baseObject, baseOffset, recordLength, sortedRecords.getKeyPrefix());
            }
            spillWriter.close();

            inMemSorter.reset();
        }

        return freeMemory();
    }

    /**
     * Return the total memory usage of this sorter, including the data pages and the sorter's pointer
     * array.
     */
    private long getMemoryUsage() {
        long totalPageSize = 0;
        for (MemoryBlock page : allocatedPages) {
            totalPageSize += page.size();
        }
        return ((inMemSorter == null) ? 0 : inMemSorter.getMemoryUsage()) + totalPageSize;
    }

    private void updatePeakMemoryUsed() {
        long mem = getMemoryUsage();
        if (mem > peakMemoryUsedBytes) {
            peakMemoryUsedBytes = mem;
        }
    }

    /**
     * Return the peak memory used so far, in bytes.
     */
    public long getPeakMemoryUsedBytes() {
        updatePeakMemoryUsed();
        return peakMemoryUsedBytes;
    }

    @VisibleForTesting
    public int getNumberOfAllocatedPages() {
        return allocatedPages.size();
    }

    /**
     * Free this sorter's data pages.
     *
     * @return the number of bytes freed.
     */
    private long freeMemory() {
        updatePeakMemoryUsed();
        long memoryFreed = 0;
        for (MemoryBlock block : allocatedPages) {
            memoryFreed += block.size();
            freePage(block);
        }
        allocatedPages.clear();
        currentPage = null;
        pageCursor = 0;
        return memoryFreed;
    }

    /**
     * Deletes any spill files created by this sorter.
     */
    private void deleteSpillFiles() {
        for (UnsafeSorterSpillWriter spill : spillWriters) {
            try {
                spill.removeFile();
            } catch (IOException e) {
                logger.error("", e);
            }
        }
    }

    /**
     * Frees this sorter's in-memory data structures and cleans up its spill files.
     */
    public void cleanupResources() {
        synchronized (this) {
            deleteSpillFiles();
            freeMemory();
            if (inMemSorter != null) {
                inMemSorter.free();
                inMemSorter = null;
            }
        }
    }

    /**
     * Checks whether there is enough space to insert an additional record in to the sort pointer
     * array and grows the array if additional space is required. If the required space cannot be
     * obtained, then the in-memory data will be spilled to disk.
     */
    private void growPointerArrayIfNecessary() throws IOException {
        assert (inMemSorter != null);
        if (!inMemSorter.hasSpaceForAnotherRecord()) {
            long used = inMemSorter.getMemoryUsage();
            LongArray array;
            try {
                // could trigger spilling
                array = allocateArray(used / 8 * 2);
            } catch (OutOfMemoryError e) {
                // should have trigger spilling
                assert (inMemSorter.hasSpaceForAnotherRecord());
                return;
            }
            // check if spilling is triggered or not
            if (inMemSorter.hasSpaceForAnotherRecord()) {
                freeArray(array);
            } else {
                inMemSorter.expandPointerArray(array);
            }
        }
    }

    /**
     * Allocates more memory in order to insert an additional record. This will request additional
     * memory from the memory manager and spill if the requested memory can not be obtained.
     *
     * @param required the required space in the data page, in bytes, including space for storing
     *                 the record size. This must be less than or equal to the page size (records
     *                 that exceed the page size are handled via a different code path which uses
     *                 special overflow pages).
     */
    private void acquireNewPageIfNecessary(int required) {
        if (currentPage == null ||
                pageCursor + required > currentPage.getBaseOffset() + currentPage.size()) {
            // TODO: try to find space on previous pages
            currentPage = allocatePage(required);
            pageCursor = currentPage.getBaseOffset();
            allocatedPages.add(currentPage);
        }
    }

    /**
     * Write a record to the sorter.
     */
    public void insertRecord(Object recordBase, long recordOffset, int length, long prefix)
            throws IOException {

        growPointerArrayIfNecessary();
        // Need 4 bytes to store the record length.
        final int required = length + 4;
        acquireNewPageIfNecessary(required);

        final Object base = currentPage.getBaseObject();
        final long recordAddress = taskMemoryManager.encodePageNumberAndOffset(currentPage, pageCursor);
        Platform.putInt(base, pageCursor, length);
        pageCursor += 4;
        Platform.copyMemory(recordBase, recordOffset, base, pageCursor, length);
        pageCursor += length;
        assert (inMemSorter != null);
        inMemSorter.insertRecord(recordAddress, prefix);
    }

    /**
     * Write a key-value record to the sorter. The key and value will be put together in-memory,
     * using the following format:
     *
     * record length (4 bytes), key length (4 bytes), key data, value data
     *
     * record length = key length + value length + 4
     */
    public void insertKVRecord(Object keyBase, long keyOffset, int keyLen,
                               Object valueBase, long valueOffset, int valueLen, long prefix)
            throws IOException {

        growPointerArrayIfNecessary();
        final int required = keyLen + valueLen + 4 + 4;
        acquireNewPageIfNecessary(required);

        final Object base = currentPage.getBaseObject();
        final long recordAddress = taskMemoryManager.encodePageNumberAndOffset(currentPage, pageCursor);
        Platform.putInt(base, pageCursor, keyLen + valueLen + 4);
        pageCursor += 4;
        Platform.putInt(base, pageCursor, keyLen);
        pageCursor += 4;
        Platform.copyMemory(keyBase, keyOffset, base, pageCursor, keyLen);
        pageCursor += keyLen;
        Platform.copyMemory(valueBase, valueOffset, base, pageCursor, valueLen);
        pageCursor += valueLen;

        assert (inMemSorter != null);
        inMemSorter.insertRecord(recordAddress, prefix);
    }

    /**
     * Merges another UnsafeExternalSorters into this one, the other one will be emptied.
     */
    public void merge(UnsafeExternalSorter other) throws IOException {
        other.spill();
        spillWriters.addAll(other.spillWriters);
        // remove them from `spillWriters`, or the files will be deleted in `cleanupResources`.
        other.spillWriters.clear();
        other.cleanupResources();
    }

    /**
     * Returns a sorted iterator. It is the caller's responsibility to call `cleanupResources()`
     * after consuming this iterator.
     */
    public UnsafeSorterIterator getSortedIterator() throws IOException {
        assert (recordComparator != null);
        if (spillWriters.isEmpty()) {
            assert (inMemSorter != null);
            readingIterator = new SpillableIterator(inMemSorter.getSortedIterator());
            return readingIterator;
        } else {
            final UnsafeSorterSpillMerger spillMerger =
                    new UnsafeSorterSpillMerger(recordComparator, prefixComparator, spillWriters.size());
            for (UnsafeSorterSpillWriter spillWriter : spillWriters) {
                spillMerger.addSpillIfNotEmpty(spillWriter.getReader());
            }
            if (inMemSorter != null) {
                readingIterator = new SpillableIterator(inMemSorter.getSortedIterator());
                spillMerger.addSpillIfNotEmpty(readingIterator);
            }
            return spillMerger.getSortedIterator();
        }
    }

    /**
     * An UnsafeSorterIterator that support spilling.
     */
    class SpillableIterator extends UnsafeSorterIterator {
        private UnsafeSorterIterator upstream;
        private UnsafeSorterIterator nextUpstream = null;
        private MemoryBlock lastPage = null;
        private boolean loaded = false;
        private int numRecords = 0;

        public SpillableIterator(UnsafeInMemorySorter.SortedIterator inMemIterator) {
            this.upstream = inMemIterator;
            this.numRecords = inMemIterator.getNumRecords();
        }

        public int getNumRecords() {
            return numRecords;
        }

        public long spill() throws IOException {
            synchronized (this) {
                if (!(upstream instanceof UnsafeInMemorySorter.SortedIterator && nextUpstream == null
                        && numRecords > 0)) {
                    return 0L;
                }

                UnsafeInMemorySorter.SortedIterator inMemIterator =
                        ((UnsafeInMemorySorter.SortedIterator) upstream).clone();

                // Iterate over the records that have not been returned and spill them.
                final UnsafeSorterSpillWriter spillWriter =
                        new UnsafeSorterSpillWriter(numRecords);
                while (inMemIterator.hasNext()) {
                    inMemIterator.loadNext();
                    final Object baseObject = inMemIterator.getBaseObject();
                    final long baseOffset = inMemIterator.getBaseOffset();
                    final int recordLength = inMemIterator.getRecordLength();
                    spillWriter.write(baseObject, baseOffset, recordLength, inMemIterator.getKeyPrefix());
                }
                spillWriter.close();
                spillWriters.add(spillWriter);
                nextUpstream = spillWriter.getReader();

                long released = 0L;
                synchronized (UnsafeExternalSorter.this) {
                    // release the pages except the one that is used. There can still be a caller that
                    // is accessing the current record. We free this page in that caller's next loadNext()
                    // call.
                    for (MemoryBlock page : allocatedPages) {
                        if (!loaded || page.getBaseObject() != upstream.getBaseObject()) {
                            released += page.size();
                            freePage(page);
                        } else {
                            lastPage = page;
                        }
                    }
                    allocatedPages.clear();
                }

                // in-memory sorter will not be used after spilling
                assert (inMemSorter != null);
                released += inMemSorter.getMemoryUsage();
                inMemSorter.free();
                inMemSorter = null;
                return released;
            }
        }

        @Override
        public boolean hasNext() {
            return numRecords > 0;
        }

        @Override
        public void loadNext() throws IOException {
            synchronized (this) {
                loaded = true;
                if (nextUpstream != null) {
                    // Just consumed the last record from in memory iterator
                    if (lastPage != null) {
                        freePage(lastPage);
                        lastPage = null;
                    }
                    upstream = nextUpstream;
                    nextUpstream = null;
                }
                numRecords--;
                upstream.loadNext();
            }
        }

        @Override
        public Object getBaseObject() {
            return upstream.getBaseObject();
        }

        @Override
        public long getBaseOffset() {
            return upstream.getBaseOffset();
        }

        @Override
        public int getRecordLength() {
            return upstream.getRecordLength();
        }

        @Override
        public long getKeyPrefix() {
            return upstream.getKeyPrefix();
        }
    }

    /**
     * Returns a iterator, which will return the rows in the order as inserted.
     *
     * It is the caller's responsibility to call `cleanupResources()`
     * after consuming this iterator.
     *
     * TODO: support forced spilling
     */
    public UnsafeSorterIterator getIterator() throws IOException {
        if (spillWriters.isEmpty()) {
            assert (inMemSorter != null);
            return inMemSorter.getSortedIterator();
        } else {
            LinkedList<UnsafeSorterIterator> queue = new LinkedList<>();
            for (UnsafeSorterSpillWriter spillWriter : spillWriters) {
                queue.add(spillWriter.getReader());
            }
            if (inMemSorter != null) {
                queue.add(inMemSorter.getSortedIterator());
            }
            return new ChainedIterator(queue);
        }
    }

    /**
     * Chain multiple UnsafeSorterIterator together as single one.
     */
    class ChainedIterator extends UnsafeSorterIterator {

        private final Queue<UnsafeSorterIterator> iterators;
        private UnsafeSorterIterator current;
        private int numRecords;

        public ChainedIterator(Queue<UnsafeSorterIterator> iterators) {
            assert iterators.size() > 0;
            this.numRecords = 0;
            for (UnsafeSorterIterator iter : iterators) {
                this.numRecords += iter.getNumRecords();
            }
            this.iterators = iterators;
            this.current = iterators.remove();
        }

        @Override
        public int getNumRecords() {
            return numRecords;
        }

        @Override
        public boolean hasNext() {
            while (!current.hasNext() && !iterators.isEmpty()) {
                current = iterators.remove();
            }
            return current.hasNext();
        }

        @Override
        public void loadNext() throws IOException {
            while (!current.hasNext() && !iterators.isEmpty()) {
                current = iterators.remove();
            }
            current.loadNext();
        }

        @Override
        public Object getBaseObject() {
            return current.getBaseObject();
        }

        @Override
        public long getBaseOffset() {
            return current.getBaseOffset();
        }

        @Override
        public int getRecordLength() {
            return current.getRecordLength();
        }

        @Override
        public long getKeyPrefix() {
            return current.getKeyPrefix();
        }
    }
}